Variable transmissivity light reflectors are well known prior art devices. Some light rays which are incident upon a variable transmissivity light reflector are partially transmitted through the reflector, some of the incident rays are reflected by the reflector and the remaining rays are absorbed by the reflector. The reflector's partially transmissive characteristic is not uniform, but varies as a function of the position at which the light rays are incident upon the reflector. In the simplest case, the reflector's transmissivity characteristic may be determined by just two values, one high and one low. For example, the high value may correspond to maximal transmission of incident light rays through the reflector (the “on” state) and the low value may correspond to minimal transmission of incident light rays through the reflector (the “off” state). The light emitting surface of a luminaire can be formed by providing a selected pattern of such on and off state reflector segments at predefined positions on the light emitting surface, with the pattern forming a simple image, such as letters for a sign. In more sophisticated cases the reflector's transmissivity characteristic may vary continuously as a function of position on the reflector, or may be a continuously varying half-tone pattern—in which case a grey scale photographic quality image can be produced on the luminaire's light emitting surface.
The two basic applications for such variable transmissivity light reflectors are luminance compensation, and production of high dynamic range static images. Luminance compensation generally involves redirection of light rays such that the rays are emitted in a preferred direction and with luminance values which vary as a selected function of position on a light emitting surface. For example, Whitehead U.S. Pat. No. 5,243,506 entitled “High Aspect Ratio Light Emitter Having High Uniformity and Directionality” employs luminance compensation to vary the degree of transmissivity of a light guide as a selected function of position to control the distribution of light emitted by the guide so as to achieve substantially uniform emission of light rays from the guide in a selected direction or within a selected angular range. Without such luminance compensation, the light guide would tend to emit light rays in a relatively nonuniform, nondirectional fashion, rendering the guide unsuitable for use in devices such as linear navigational beacons, which preferably emit maximum light intensity in a substantially horizontal direction; certain backlit liquid crystal displays, which preferably emit light only within a desired range of viewing angles; and certain vehicle signal lights, which preferably emit maximum light intensity only in desired directions.
To illustrate the luminance compensation problem, FIG. 1 depicts a typical prior art light box 10 of the type used in advertising signs. The interior of light box 10 contains and is illuminated by a plurality of fluorescent tubes 12, only two of which are shown. Light box 10's inside rearward surface 14 and inside side surfaces 16, 18 are coated or lined with a reflective material such as white paint or reflective film, it being understood that the best available prior art materials have intrinsic reflectance values of about 90%.
Light box 10's light emitting image display surface 20 has a variable transmissivity characteristic which varies as a function of position over light emitting surface 20. The particular variable transmissivity characteristic is selected to suit the image to be displayed on the outside of light emitting surface 20. That characteristic may be produced in a manner well known to persons skilled in the art, for example as explained in Whitehead U.S. Pat. Nos. 6,024,462 and 6,079,844 which are both titled “High Efficiency High Intensity Backlighting of Graphic Displays.” For example, light emitting surface 20 may incorporate a perforated reflective material—it again being understood that the best available prior art materials have intrinsic reflectance values no greater than about 90%.
The width W of light box 10 (i.e. the displacement between rearward surface 14 and light emitting image display surface 20) must not be less than a predetermined minimum value—typically, the ratio of the width W of box 10 compared to the centre-to-centre spacing S between adjacent fluorescent tubes 12, where W/S is of order 1. Otherwise, an unacceptably large fraction of the light rays emitted by each fluorescent tube 12 will illuminate only a relatively small region 22 of light emitting surface 20 immediately adjacent the particular fluorescent tube. Due to the relatively low intrinsic reflectance value of the material incorporated in light emitting surface 20, an unacceptably large fraction of the light rays which illuminate regions 22 are absorbed by light emitting surface 20 and “lost.” That is, such “lost” rays are neither transmitted through light emitting surface 20 to illuminate the displayed image, nor are they reflected by light emitting surface 20 back toward rearward surface 14 for further reflection and eventual transmission through some other region on light emitting surface 20.
Regions 22 typically overlap portions of the image to be displayed on light emitting surface 20. The variable transmissivity characteristic of light emitting surface 20 is accordingly selected to permit an appropriate fraction of light rays incident upon regions 22 to escape through light emitting surface 20 to illuminate the image. But the aforementioned loss of light rays due to absorption leaves insufficient light to be reflected for eventual transmission through some other region on light emitting surface 20. Such other regions are accordingly not illuminated to the same extent as regions 22. Consequently, observers perceive regions 22 as over-illuminated bright spots, which is undesirable. One prior art solution to this problem is to increase the width W of light box 10 to broaden regions 22 as shown in FIG. 2 and thereby reduce the perceptibility of bright spots on light emitting surface 20. However this unavoidably increases the size of light box 10, which is undesirable. Another prior art solution to the foregoing problem is to adjust the variable transmissivity characteristic of light emitting surface 20 to reduce the light transmission capability of light emitting surface 20 in each of regions 22, while making corresponding adjustments to the variable transmissivity characteristic of light emitting surface 20 outside regions 22. Such adjustment involves a cumbersome, time-consuming, iterative trial and error technique requiring a custom solution for every different light box (and for every different high dynamic range image). This application addresses the foregoing problem.
This application also discloses display of high dynamic range images. Dynamic range is the ratio of intensity of the highest and lowest luminance parts of a scene. For example, the image projected by a video projection system may have a maximum dynamic range of 300:1. This relatively low dynamic range is due to the relatively limited range of luminance values which can be reproduced by a typical video projection system. By contrast, the human visual system is capable of recognizing features in scenes which have very high dynamic ranges. For example, a person can look into the shadows of an unlit garage on a brightly sunlit day and see details of objects in the shadows, even though the luminance in adjacent sunlit areas may be tens of thousands of times greater than the luminance in the shadow parts of the scene.
There are many high dynamic range image situations which the human eye can perceive well, but which cannot be effectively displayed due to the dynamic range limitations of conventional image display systems. Examples include most situations where sources of light are in the field of view, such as sunset scenes, scenes containing highly reflective (“shiny”) surfaces, or night scenes containing illuminated neon signs, lamps, etc. The ability to display a larger dynamic range of luminance values would facilitate production of more visually effective graphic images, such as scenes of the aforementioned type which contain sources of light. This would in turn have value both aesthetically and in more effective advertising. However, to display a realistic rendering of a scene of the foregoing type can require a display having a dynamic range in excess of 1000:1. In this specification, the term “high dynamic range” means dynamic ranges of 800:1 or more.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.